#lang racket
(provide rember firsts)
(define rember
  (lambda (a lat) 
    (cond
      ((null? lat) '())
      ((eq? (car lat) a)  (cdr lat))
      (else (cons (car lat) (rember a (cdr lat))))
      )
    )
  )

(define firsts
  (lambda (l)
    (cond
      ((null? l)'())
      (else (cons (car (car l)) (firsts (cdr l)) ))
      )
    )
  )